Alot of people will tell you bad things about Powerflow dealers, how many of these have had first hand expeience with Powerflow is unclear.
I shouldnt see why they would have problems with a simple straight through centre section but before you make a booking, check on
www.kam-racing.com as Supersprint do two straight through centre sections for the saxo one is just a simple boxless pipe the other also replaces the cat.
Look into prices as you could end up paying more for a custom job when you could have bought a direct fitting item for less.
Hope that helps